Create directory based on part of filename - regex

Question

First of all, I'm not a programmer — just trying to learn the basics of shell scripting and trying out some stuff.
I'm trying to create a function for my bash script that creates a directory based on a version number in the filename of a file the user has chosen in a list.
Here's the function:
lav_mappe () {
shopt -s failglob
echo "[--- Choose zip file, or x to exit ---]"
echo ""
echo ""
select zip in $SRC/*.zip
do
[[ $REPLY == x ]] && . $HJEM/build
[[ -z $zip ]] && echo "Invalid choice" && continue
echo
grep ^[0-9]{1}\.[0-9]{1,2}\.[0-9]{1,2}$ $zip; mkdir -p $MODS/out/${ver}
done
}
I've tried messing around with some other commands too:
for ver in $zip; do
grep "^[0-9]{1}\.[0-9]{1,2}\.[0-9]{1,2}$" $zip; mkdir -p $MODS/out/${ver}
done
And also find | grep — but I'm doing it wrong :(
But it ends up saying "no match" for my regex pattern.
I'm trying to take the filename the user has selected, then grep it for the version number (ALWAYS x.xx.x somewhere in the filename), and fianlly create a directory with just that.
Could someone give me some pointers what the command chain should look like? I'm very unsure about the structure of the function, so any help is appreciated.
EDIT:
Ok, this is how the complete function looks like now: (Please note, the sed(1) commands besides the directory creation is not created by me, just implemented in my code.)
Pastebin (Long code.)

Answer 1

I've got news for you. You are writing a Bash script, you are a programmer!
Your Regular Expression (RE) is of the "wrong" type. Vanilla grep uses a form known as "Basic Regular Expressions" (BRE), but your RE is in the form of an Extended Regular Expression (ERE). BRE's are used by vanilla grep, vi, more, etc. EREs are used by just about everything else, awk, Perl, Python, Java, .Net, etc. Problem is, you are trying to look for that pattern in the file's contents, not in the filename!
There is an egrep command, or you can use grep -E, so:
echo $zip|grep -E '^[0-9]\.[0-9]{1,2}\.[0-9]{1,2}$'
(note that single quotes are safer than double). By the way, you use ^ at the front and $ at the end, which means the filename ONLY consists of a version number, yet you say the version number is "somewhere in the filename". You don't need the {1} quantifier, that is implied.
BUT, you don't appear to be capturing the version number either.
You could use sed (we also need the -E):
ver=$(echo $zip| sed -E 's/.*([0-9]\.[0-9]{1,2}\.[0-9]{1,2}).*/\1/')
The \1 on the right means "replace everything (that's why we have the .* at front and back) with what was matched in the parentheses group".
That's a bit clunky, I know.
Now we can do the mkdir (there is no merit in putting everything on one line, and it makes the code harder to maintain):
mkdir -p "$MODS/out/$ver"
${ver} is unnecessary in this case, but it is a good idea to enclose path names in double quotes in case any of the components have embedded white-space.
So, good effort for a "non-programmer", particularly in generating that RE.
Now for Lesson 2
Be careful about using this solution in a general loop. Your question specifically uses select, so we cannot predict which files will be used. But what if we wanted to do this for every file?
Using the solution above in a for or while loop would be inefficient. Calling external processes inside a loop is always bad. There is nothing we can do about the mkdir without using a different language like Perl or Python. But sed, by it's nature is iterative, and we should use that feature.
One alternative would be to use shell pattern matching instead of sed. This particular pattern would not be impossible in the shell, but it would be difficult and raise other questions. So let's stick with sed.
A problem we have is that echo output places a space between each field. That gives us a couple of issues. sed delimits each record with a newline "\n", so echo on its own won't do here. We could replace each space with a new-line, but that would be an issue if there were spaces inside a filename. We could do some trickery with IFS and globbing, but that leads to unnecessary complications. So instead we will fall back to good old ls. Normally we would not want to use ls, shell globbing is more efficient, but here we are using the feature that it will place a new-line after each filename (when used redirected through a pipe).
while read ver
do
mkdir "$ver"
done < <(ls $SRC/*.zip|sed -E 's/.*([0-9]{1}\.[0-9]{1,2}\.[0-9]{1,2}).*/\1/')
Here I am using process substitution, and this loop will only call ls and sed once. BUT, it calls the mkdir program n times.
Lession 3
Sorry, but that's still inefficient. We are creating a child process for each iteration, to create a directory needs only one kernel API call, yet we are creating a process just for that? Let's use a more sophisticated language like Perl:
#!/usr/bin/perl
use warnings;
use strict;
my $SRC = '.';
for my $file (glob("$SRC/*.zip"))
{
$file =~ s/.*([0-9]{1}\.[0-9]{1,2}\.[0-9]{1,2}).*/$1/;
mkdir $file or die "Unable to create $file; $!";
}
You might like to note that your RE has made it through to here! But now we have more control, and no child processes (mkdir in Perl is a built-in, as is glob).
In conclusion, for small numbers of files, the sed loop above will be fine. It is simple, and shell based. Calling Perl just for this from a script will probably be slower since perl is quite large. But shell scripts which create child processes inside loops are not scalable. Perl is.